Black Butte's dark mystery explained

South of Weed, Calif., along Interstate 5 at Black Butte Summit stands the impressive but silent sentinel of Black Butte. It just has to be a cinder cone, right?

By Jad D'Allura

South of Weed, Calif., along Interstate 5 at Black Butte Summit stands the impressive but silent sentinel of Black Butte. It just has to be a cinder cone, right?

Like many absolute certainties in life, that supposition just isn't correct.

Black Butte, which formed 9,600 years ago, consists of a series of four closely nested dacite lava domes mantled by huge boulder-debris aprons. It is a satellite eruption associated with Mount Shasta, which looms like a watchful giant to the east.

Why isn't it a cinder cone? Cinder cones are composed of frothy red or black, pebble-sized to cobble-sized lava fragments spewed like a Roman candle from a central vent area. The larger particles collect around the vent to form a symmetrical cone of loose fragments with 30- to 35-degree slopes. Dacite domes form when very sticky, gas-poor blobs of lava are pushed up from the ground like blunt spears. These stubby domes are creased and cracked, looking like Darth Vader's head after his helmet was removed.

Dacite is a high-silica (quartz-rich) lava with abundant calcium. The high silica content makes the lava more viscous (stiff), so it builds stubby flows or domes. There are no cinders on Black Butte, only light gray boulders. The lava isn't strong, hence it cracks and crumbles like brittle toothpaste when it's extruded to the surface. Crumbling debris builds the steep-sided boulder aprons that stream down from the four nested domes. Because of the large size of the boulders, the slopes of dacite domes are steeper than those of cinder cones.

To see dacite boulders, from I-5 take the off-ramp at Truck Village Road (Exit 743). Park carefully and examine rocks that form the base of the road leading up and away from Truck Village. When fresh, this dacite is a beautiful, light-gray rock with lustrous, elongated black crystals of hornblende. The pinkish "wash" that permeates some boulders is due to iron oxide weathering. The dark color of Black Butte is attributed to lichens that adorn (adore?) the rock surfaces.

If you're interested, there's a hiking trail leading to the summit on the east side of the butte.

A true cinder cone exists east of I-5 between exits 741 and 740. It's a tree- and brush-covered cone with a gentler slope. It looks like a comfortable old hat with a dimple in the center that marks the main vent area. Vegetation masks the red and black colors of the cinders, but those can be seen along the frontage road.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.